Semi-active dampers may be of the "off/on" type or of the "continuously" or "infinitely" variable type. A damper of the first type is switched, in accordance with the dictates of a suitable control policy, between alternative "on" and "off" damping states or conditions. In its on state, the damping coefficient and corresponding damping force of the damper is of a preselected relatively high magnitude. The term "damping coefficient," as used herein, means the relationship of the damping force generated by the damper to the relative velocity across the damper, which relationship is not necessarily linear. In its off state, the damping coefficient and the corresponding damping force of the damper is of relatively low magnitude. This may be approximately zero, but in many primary vehicle suspensions should be of a magnitude sufficiently greater than zero as to discourage "wheel hop." A continuously variable semi-active damper is also switched during operation between an off state, wherein its damping coefficient and corresponding damping force is approximately zero or of other low magnitude, and an on state. However, when a continuously variable damper is in its on state the damping coefficient and corresponding damping force thereof may be and normally is changed between a large (theoretically infinite) number of different magnitudes. If operated pursuant to a suitable control policy, a continuously variable semi-active damper may be caused to perform, when in its on state, in a manner similar to the hypothetical "sky-hook" damper discussed in an article by M. J. Crosby et al., entitled "Vibration Control Using Semi-Active Force Generators" and in U.S. Pat. No. 4,742,998.
A known control policy for a continuously variable semi-active damper dictates that the damper be "on," and that the damping forces generated by it be proportional (although not necessarily linearly) to the absolute velocity of the supported member, when the sign of the product of such absolute velocity times the relative velocity between the supported and supporting members is positive, i.e., greater than zero. Contrarily, the policy dictates that the damper be set to its off state, in which the damping coefficient and damping force is of preselected low magnitude, when the sign of the aforesaid product is negative, i.e., when the product is less than zero. Generally comparable results may be achieved, particularly at relatively high frequency excitations, by use of an alternative control policy. The alternate control dictates that damping forces produced by the continuously variable semi-active damper be proportional to the relative displacement between the supported and supporting members at those times when the product of the relative velocity times the relative displacement between the members is less than zero, i.e., when the sign of the product is negative; and that the damping forces be of a low magnitude when the aforesaid product is greater than zero, i.e., when its sign is positive or plus.
Although generally producing good results, vibration attenuating systems having continuously variable semi-active damper means controlled in strict accordance with the control policies of the foregoing or similar types may experience shock forces of significant magnitude, i.e., a jerky feel, at some of the times when the damper is switched between its different damping states or conditions, due to system delays, estimation of control signals, or both. The aforesaid shocks may stress system components to such an extent as to shorten their useful life, and/or may cause the generation of objectionable noise or vibration. The problem of noise generation may be particularly apparent in automobile suspensions or other systems containing a resilient deformable member, such as an automobile tire, that is capable of storing energy upon deformation, and of abruptly releasing its stored energy when allowed to rapidly return toward an undeformed condition.
FIG. 1 is a block diagram of a known "skyhook" control policy for semi-active suspensions systems. This control policy requires two inputs and generates a single control output to drive a semi-active device, such as a controllable damper. The two inputs are absolute velocity (V.sub.abs) on one side of the device and relative velocity (V.sub.rel) across the device. These inputs may be sensed directly, estimated (integrated or differentiated) directly from sensor measurements, or estimated from sensor measurements and a dynamic system model.
The absolute velocity V.sub.abs is scaled by a positive factor gain G' in a gain block 13'. The absolute velocity V.sub.abs is also multiplied by the relative velocity signal V.sub.rel in a multiplier block 12a' to form the velocity product V.sub.abs *V.sub.rel. The velocity product signal is input to a logical test block 14'; the output of which is unity (1.0) or "true" when the input is positive, and zero or "false" otherwise. The output of the logical test block 14' is a gating signal 16' which is multiplied by the scaled absolute velocity signal 15' in a multiplier block 12b' to determine the appropriate desired damper force Fdesired'.
FIG. 2 is a three-dimensional control surface plot of desired damper force Fdesired' as a function of the two inputs V.sub.abs and V.sub.rel for the standard skyhook control policy as described above. Notably, a surface discontinuity 17' is present in the control surface 11' at V.sub.rel =0. This surface discontinuity 17' may lead to an undesirable "jerk" or a "nervous feel" which may be experienced by passengers. FIG. 3 shows graphically (supported mass acceleration versus time) experimental data from a suspension system implementing the aforedescribed known "skyhook" control policy. Notably, the sharp vertical peaks 19 (only several of which are labeled) represented in the graph reflect behavior of the suspension system which may be experienced as "jerking."
U.S. Pat. No. 4,887,699 discloses a method for reducing the generation of undesirable shock forces and/or noise tending to occur in some vehicle suspension systems, or in other mounting systems, having at least one semi-active damper of the continuously variable type. Operation of the continuously variable damper is modified to include delaying some or all of the changes in the on/off states of the damper, and/or limiting the "rate and/or extent" of changes in the damping coefficient and thus the corresponding damping force of the damper at certain of the times when the damper is in an on state.
FIG. 4 is a three-dimensional plot of a control surface 11" illustrating the desired damper force Fdesired" of an exemplary control policy according to U.S. Pat. No. 4,887,699 as a function of the absolute velocity V.sub.abs of the supported mass and the relative velocity V.sub.rel across the suspension. Notably, the control surface 11", as shown, includes "creases" , i.e., a slope discontinuity 18", in the quadrants where V.sub.abs *V.sub.rel &gt;0. These "creases" or slope discontinuities in the Fdesired" control surface 11" may manifest as jerkiness similar to that experienced at V.sub.rel =0 due to the surface discontinuity 17' for the aforedescribed standard skyhook control policy of